Introduction to Complex Numbers Complex Numbers

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Dinah Mosley
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Introduction to SUGGESTED LEARNING STRATEGIES: Summarize/Paraphrase/ Retell, Activating Prior Knowledge, Create Representations The equation x 2 + 1 = 0 has special historical and mathematical

Why would mathematicians of the early sixteenth century think that x 2 + 1 = 0 had no solutions? ACTIVITY 3.

In the process of solving one cubic (third degree) equation, he encountered and was required to make use of the square roots of negative numbers.

1 Introduction to SUGGESTED LEARNING STRATEGIES: Summarize/Paraphrase/ Retell, Activating Prior Knowledge, Create Representations The equation x = 0 has special historical and mathematical significance. At the beginning of the sixteenth century, mathematicians believed that the equation had no solutions. 1. Why would mathematicians of the early sixteenth century think that x = 0 had no solutions? ACTIVITY 3.3 A breakthrough occurred in 1545 when the talented Italian mathematician Girolamo Cardano ( ) published his book, Ars Magna (The Great Art). In the process of solving one cubic (third degree) equation, he encountered and was required to make use of the square roots of negative numbers. While skeptical of their existence, he demonstrated the situation with this famous problem: Find two numbers with the sum 10 and the product To better understand this problem, first find two numbers with the sum 10 and the product Letting x represent one number, write an expression for the other number in terms of x. Use the expressions to write an equation that models the problem in Item Solve your equation in Item 3 in two different ways. Explain each method. Unit 3 Quadratic Functions and 157

2 ACTIVITY 3.3 Introduction to SUGGESTED LEARNING STRATEGIES: Create Representations, Activating Prior Knowledge, Interactive Word Wall, Marking the Text 5. Write an equation that represents the problem that Cardano posed. CONNECT TO HISTORY When considering his solutions, Cardano dismissed mental tortures and ignored the fact that x x = x only when x 0. _ 6. Cardano claimed that the solutions to the problem are x = _ and x = Verify his solutions by using the Quadratic Formula with the equation in Item 5. ACADEMIC VOCABULARY An imaginary number is any number of the form bi, where b is a real number and i = -1. CONNECT TO HISTORY René Descartes ( ) was the first to call these numbers imaginary. Although his reference was meant to be derogatory, the term imaginary number persists. Leonhard Euler ( ) introduced the use of i for the imaginary unit. Cardano avoided any more problems in Ars Magna involving the square root of a negative number. However, he did demonstrate an understanding about the properties of such numbers. Solving the equation x = 0 yields the solutions x = -1 and x = The number -1 is represented by the symbol i, the imaginary unit. You can say i = -1. The imaginary unit i is considered the solution to the equation i = 0, or i 2 = -1. To simplify an imaginary number -s, where s is a positive number, _ you can write -s = i s. 158 SpringBoard Mathematics with Meaning TM Algebra 2

3 Introduction to ACTIVITY 3.3 Knowledge, Interactive Word Wall, Marking the Text EXAMPLE 1 _ Write the numbers -17 and -9 in terms of i. Step 1: Definition of _ s = i 17 = i 9 Step 2: Take the square root of 9. = i 17 = i 3 = 3i WRITING MATH Write i 17 instead of 17 i, which may be confused with 17i. TRY THESE A Write each number in terms of i. _ a. -25 b. -7 _ c. -12 d _ 7. Write the solutions to Cardano s problem, x = and _ x = , using the imaginary unit i. The set of complex numbers consists of the real numbers and the imaginary numbers. A complex number is a number in the form a + bi, where a and b are real numbers and i = -1. Each complex number has two parts: the real part a and the imaginary part b. For example, in 2 + 3i, the real part is 2 and the imaginary part is 3. TRY THESE B Identify the real part and the imaginary part of each complex number. a i b. 8 ACADEMIC VOCABULARY complex number c. i 10 d i 2 Unit 3 Quadratic Functions and 159

4 ACTIVITY 3.3 Introduction to Knowledge, Group Presentation 8. Using the definition of complex numbers, show that the set of real numbers is a subset of the set of complex numbers. Perform addition of complex numbers as you would for addition of binomials of the form a + bx. To add such binomials, you collect like terms. EXAMPLE 2 Step 1 Collect like terms. Addition of Binomials Addition of Complex Numbers (5 + 4x) + (-2 + 3x) (5 + 4i) + (-2 + 3i) = (5-2) + (4x + 3x) = (5-2) + (4i + 3i) Step 2 Simplify. = 3 + 7x = 3 + 7i TRY THESE C Add the complex numbers. a. (6 + 5i) + (4-7i) b. (-5 + 3i) + (-3 - i) c. (2 + 3i) + (-2 + 3i) 9. Use Example 2 above and your knowledge of operations of real numbers to write general formulas for the sum and difference of two complex numbers. (a + bi) + (c + di) = (a + bi) - (c + di) = 160 SpringBoard Mathematics with Meaning TM Algebra 2

5 Introduction to ACTIVITY 3.3 Knowledge, Group Presentation 10. Find each sum or difference of the complex numbers. a. (12-13i) - (-5 + 4i) b. ( i ) + ( i ) c. ( 2-7i) + (2 + i 3 ) d. (8-5i) - (3 + 5i) + ( i) Perform multiplication of complex numbers as you would for multiplication of binomials of the form a + bx. The only change in procedure is to replace i 2 with -1. EXAMPLE 3 Multiply Binomials (2 + 3x)(4-5x) 2(4) + 2(-5x) + 3x(4) + 3x(-5x) 8-10x + 12x - 15 x x - 15 x 2 TRY THESE D Multiply the complex numbers. a. (6 + 5i)(4-7i) b. (2-3i)(3-2i) c. (5 + i)(5 + i) Multiply (2 + 3i)(4-5i) 2(4) + 2(-5i) + 3i(4) + 3i(-5i) 8-10i + 12i - 15 i i - 15 i 2 Now substitute -1 for i i - 15 i 2 = 8 + 2i - 15(-1) = i Unit 3 Quadratic Functions and 161

6 ACTIVITY 3.3 Introduction to Since -1 = i, the powers of i can be evaluated as follows: i 1 = i i 2 = -1 i 3 = i 2 i = -1i = -i i 4 = i 2 i 2 = (-1) 2 = 1 Since i 4 = 1, further powers repeat the pattern shown above. i 5 = i 4 i = i i 6 = i 4 i 2 = i 2 = -1 i 7 = i 4 i 3 = i 3 = -i i 8 = i 4 i 4 = i 4 = 1 ACADEMIC VOCABULARY complex conjugate Knowledge, Look for a Pattern, Group Presentation 11. Use Example 3 and your knowledge of operations of real numbers to write a general formula for the multiplication of two complex numbers. (a + bi) (c + di) = 12. Use operations of complex numbers to verify that the two solutions that Cardano found, x = and x = , have a sum of 10 and a product of 40. The complex conjugate of a complex number a + bi is defined as a - bi. For example, the complex conjugate of 2 + 3i is 2-3i. 13. A special property of multiplication of complex numbers occurs when a number is multiplied by its conjugate. Multiply each number by its conjugates then describe the product when a number is multiplied by its conjugate. a. 2-9i b i 14. Write an expression to complete the general formula for the product of a complex number and its complex conjugate. (a + bi)(a - bi) = 162 SpringBoard Mathematics with Meaning TM Algebra 2

7 Introduction to ACTIVITY 3.3 Knowledge, Group Presentation, Interactive Word Wall, Marking the Text EXAMPLE 4 Divide i 3i. Step 1: Multiply the numerator and denominator by the complex conjugate. Step 2: Simplify and substitute -1 for i 2. Step 3: Simplify and write in the form a + bi. 4-5i 2 + 3i = i 3i = 8-22i + 15 i 2 4-6i + 6i - 9i 2 = 8-22i = -7-22i 13 (2-3i) _ (2-3i) = i TRY THESE E a. In Example 4, why is the quotient i equivalent to the original 13 expression i 3i? Divide the complex numbers. Write your answers on notebook paper. Show your work. b. 5i 2 + 3i c i d. _ 1 - i 3-4i 3 + 4i 15. Use Example 4 and your knowledge of operations of real numbers to write a general formula for the division of two complex numbers. (a + bi) _ (c + di) = Complex numbers in the form a + bi can be represented geometrically as points in the complex plane. The complex plane is a rectangular grid, similar to the Cartesian Plane, with the horizontal axis representing the real part a of a complex number and the vertical axis representing the imaginary part b of a complex number. The point (a, b) on the complex plane represents the complex number a + bi. TECHNOLOGY Many graphing calculators have the capability to perform operations on complex numbers. Unit 3 Quadratic Functions and 163

8 ACTIVITY 3.3 Introduction to Knowledge, Create Representations EXAMPLE 5 Point A represents 0 + 4i. Point B represents i. Point C represents 1-4i. Point D represents 3 + 0i. B imaginary axis 4 2 A 5 D 5 2 real axis 4 C TRY THESE F a. Graph 2 + 3i and -3-4i on the complex plane above. Graph each complex number on a complex plane grid. b i c. 4-3i d i e. -2i f. -5 CHECK YOUR UNDERSTANDING Write your answers on notebook paper or grid paper. Show your work. 1. Write each expression in terms of i. a. -49 b. -13 _ c d These quadratic equations have solutions that are complex. Use the quadratic formula to solve the equations. a. x 2 + 3x + 4 = 0 b. 2 x 2-6x + 9 = 0 3. Find each sum or difference. a. (6-5i) + (-2 + 6i) b. (4 + i) + (-4 + i) c. (5-3i) - (3-5i) d. (-3 + 8i) - ( i ) 4. Multiply. Write each product in the form a + bi. a. (2 + 9i)(3 - i) b. (-5 + 8i)(2 - i) c. (8 + 15i)(8-15i) 5. Divide. Write each quotient in the form a + bi. a i b. _ i c. 4 - i 3-4i 7 - i 3i 6. Draw the complex plane. Then graph each complex number on the plane. a. 6i b i c. -2-5i d. 4 - i e i 7. MATHEMATICAL REFLECTION equations conjugates? Why are complex number solutions of quadratic 164 SpringBoard Mathematics with Meaning TM Algebra 2

Factors of Polynomials Factoring For Experts

Factors of Polynomials Factoring For Experts Factors of Polynomials SUGGESTED LEARNING STRATEGIES: Shared Reading, Activating Prior Knowledge, Discussion Group, Note-taking When you factor a polynomial, you rewrite the original polynomial as a product